1. Field of the Invention
This invention relates to variable gain amplifier circuits and more particularly to non-attenuating voltage-controlled automatic variable gain amplifier circuits.
2. Description of the Related Art
Analog signal processing systems often receive input signals with variable amplitudes spread over a wide bandwidth and require that signal's amplitude to be controlled. Variable gain amplifier (VGA) circuits with manual or automatic gain control provide this function. Of particular importance is the VGA circuit's capacity to preserve the whole range of amplitudes received and prevent small input signals from being reduced to the noise level when the input signal has both very high and very low amplitudes.
A number of present VGA circuits are open-loop. A problem with open-loop VGAs is that they do not have the corrective benefits of a feedback system and, thus, are prone to producing high degrees of input to output distortion, especially as their operating temperature rises. Furthermore, there are non-linearities associated with an open-loop circuit which result in a non-linear transfer function between the input and output signals. Also, an open-loop VGA circuit's signal bandwidth is dependent upon the circuit's variable gain, which produces the undesirable result of a variable bandwidth. The power consumed by open-loop VGA circuits can also-be high.
Another type of VGA circuit is disclosed in U.S. Pat. No. 5,077,541 to Gilbert, Dec. 31, 1991. The circuit has three basic components: 1) a resistive attenuator, to which the input signal is applied; 2) a circuit to set the attenuation factor o the attenuator to any value within a continuous range of values; and 3) a fixed gain amplifier for receiving the attenuated signal and producing the final output. The attenuator is a ladder network of resistors between the circuit's input voltage and ground. The attenuator is tapped at each ladder step by a gain-control circuit, which comprises a set of current-controllable transconductance (g.sub.m) stages, and continuously interpolates between and along the ladder steps (similar to a potentiameter).
A steerable control current sequentially varies the transconductance of each g.sub.m stage in a smoothly changing manner to activate or de-activate the g.sub.m stages in an over-lapping sequence. The outputs of each g.sub.m stage are provided at a common node where they are summed and the summed signal is provided as an input to the fixed gain amplifier. The fixed gain amplifier provides the VGA output.
The problem with this type of VGA circuit is that, by initially attenuating the input signal, some smaller signals may be attenuated down to or close to the fixed gain amplifier's input referred noise level. Small input signals will then be lost or significantly corrupted by any noise power and the output will not resemble the input. This is especially troublesome when one input signal dominates the total power in the input signal path.
Another existing amplifier circuit includes the fixed gain feedback operation amplifier (op amp) circuit. This circuit has a fixed value feedback resistor connected between the output of the op amp and its inverting input. A fixed value gain setting resistor is connected between the op amp's inverting input and ground with the signal to be amplified received at the op amp's non-inverting input. The feedback op amp circuit is described in Grebene, Bipolar and MOS Analog Integrated Circuit Design, John Wiley & Sons, Inc., 1984, pages 310-314. The fixed gain of the feedback op amp circuit is one plus the ratio of the feedback resistor to the gain setting resistor. A problem with fixed gain amplifier circuits that they are not functional when a variable gain is required.